The present invention relates to a liquid crystal display element. In more detail, it relates to a technique for avoiding a phenomenon occurred when a transparent electrode composed of ITO is observed in a reflecting state.
Generally, ITO (indium-tin-oxide) is used for a transparent electrode utilized in a liquid crystal display element, and an electrode pattern is formed by patterning a conductive ITO film. The refractive index of ITO is about 1.9 which is larger than the refractive index of about 1.5 of a glass substrate as a supporting substrate.
Accordingly, the presence or absence of ITO provides a different intensity of reflection when outside light enters into the display element. In this case, the transparent ITO-pattern becomes visible whereby the quality of display is deteriorated remarkably. This phenomenon is called xe2x80x9cITO bone appearancexe2x80x9d which is also called xe2x80x9cvisible electrodesxe2x80x9d.
As a technique for avoiding the ITO bone appearance, there has been known to adjust optimally the refractive index and the thickness of the insulation film formed on ITO and the refractive index and the thickness of an aligning film, wherein the optimum condition can be obtained by theoretical calculation. Since the phenomenon of ITO bone appearance can be treated theoretically as the reflection of light with respect to a multilayer film having different refractive indices, it is possible to understand the characteristics according to calculation of the reflection of a generally used multilayer film. Further, it is confirmed that a calculation result coincides with an experimental result in general.
In many cases, a TN type liquid crystal wherein liquid crystal molecules are twisted 90xc2x0 is used for a liquid crystal display. This TN type liquid crystal includes a type of liquid crystal display called MTN (modulated twisted nematic) which improves non-coloration characteristics of white and black. This type concerns a method for providing a distribution of gap for the liquid crystal layer.
As the method for providing a distribution of gap for the liquid crystal layer, there has been known a method for conducting a roughening treatment to a glass substrate to form a concave-convex portion. For example, when an etching treatment is conducted using HF (hydrogen fluoride), there is obtainable a concave-convex portion of a level in which the pitch of peak to peak is about 100 xcexcm and the depth of peak to bottom is about 5.5 xcexcm.
The method for forming a film on the concave-convex portion of the glass substrate is classified generally into a sputtering method using a drying process and a flexographic printing method or spin coating method which belongs to a solvent coating type. In the sputtering method as the former case, there are problems that a usable device is expensive; it takes much time to form the film, and material other than an inorganic material can not be used for the film although there is an advantage that a film thickness distribution does not substantially result in the film formed. Accordingly, the former case is unsuitable for practical use.
On the other hand, the flexographic printing method or spin coating method as the latter case is employed in many production processes because a usable device is inexpensive; it is possible to form the film in a shorter time, and an organic material can be used. However, it is known that the film formed has a film thickness distribution along the concave-convex portion.
As described above, it is possible to obtain the optimum film thickness of each structural element according to the theoretical calculation, whereby it is possible to avoid the ITO bone appearance. As a matter of fact, however, there was such disadvantageous as follows. When an insulation film was formed by using a solvent coating method on a concave-convex portion of the substrate of the above-mentioned MTN liquid crystal display type, the optimum film thickness could not be obtained unlike the result obtainable according to the theoretical calculation because a film thickness distribution took place along the concave-convex portion, whereby it was difficult to reduce the ITO bone appearance.
For the reason described above, it is not preferable to use the sputtering method although it provides a predetermined uniform film thickness.
The present invention, accordingly, aims at avoiding a phenomenon of ITO bone appearance without causing an increase of manufacturing cost and a reduction of productivity, and provides a liquid crystal display element of, in particular, a MTN liquid crystal display type, whereby both the improvement of non-coloration characteristics of white and black and the avoidance of the phenomenon of ITO bone appearance can be satisfied.
In order to solve the above-mentioned problems, the present invention provides a liquid crystal display element comprising a pair of supporting substrates each having a transparent electrode patterned in a predetermined shape and an insulation film formed on at least one transparent electrode at a side of liquid crystal, wherein the insulation film is a film which provides a refractive index difference of not more than 0.15 with respect to the refractive index of the transparent electrode and has a film thickness distribution of at least xc2x1500 xc3x85 in the area of a radius of 500 xcexcm.
In the above-mentioned present invention, it is preferable that a concave-convex portion is formed on the supporting substrate at a side where the transparent electrode is formed, so as to provide a film thickness distribution to the insulation film. Further, in order to solve the above-mentioned problems more efficiently, it is preferable that the depth of peak to bottom of the concave-convex portion is at least 3 xcexcm.
In a liquid crystal display element of a type that the insulation film is not required, the aligning film should have the above-mentioned refractive index and film thickness distribution.